Sophomore Clinic ENGR 01-202 5, CRN 20686 Introduction to PIC Programming in C

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Sophomore Clinic ENGR 01-202 5, CRN
20686Introduction to PIC Programming in C

• James K. Beard, Ph.D.

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Topics

• Requirements
• What do we need to do?
• What do we have to accomplish this?
• PIC 16F876A Capabilities
• The Program Issues
• The PWM
• The rest of the program
• The C Program Code
• Issues
• What do you do?
• What can you do to improve your project once it
  is all working?

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Basic Requirements

• Notional Functions (Your design may differ)
• Drive H-Bridges for Two bi-directional DC motors
• Drive Electromagnet On-Off
• Controls on HMI
• Two potentiometers on thumbwheels
• Three pushbuttons
• Use a PIC 16F876A
• Three buses
• A Bus for up to five analog signals
• B and C Bus are eight bits bi-directional
• 4 MHz clock
• C Code for the PIC using CCS PIC C

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PIC 16F876A Capabilities

• Mid-range PIC
• 28 Pins, low power, RISC instruction set, slow
  arithmetic
• Low end is 18 pins and smaller
• High end is 40 pins, fast arithmetic
• Five A to D Converters
• Two Pulse Width Modulators (PWMs)
• 23 Programmable I/O Pins
• Much other stuff that we dont need for SC
• Just right for our project

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The PIC 16F876A
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PIC 16F876A Buses

• Bus A
• Six individually programmable I/O lines
• Analog or digital inputs and digital outputs
• Up to 5 ADC inputs
• Programmable pull-ups for switched inputs
• Bus B
• Eight individually programmable digital I/O lines
• Programmable pull-ups
• Bus C eight individually programmable I/O lines

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Devices in the PIC 16F876A

• Processor
• Memory
• 8 K of 14-bit instruction flash memory
• 368 bytes of program memory
• 256 bytes of EEPROM
• Five 10-bit ADCs
• Some configurable with references
• One configurable with both high and low reference
• Two oscillators
• Backup 2.5 MHz R-C clock oscillator
• Quartz clock up to 10 MHz

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Devices (Continued)

• RS-232 mapped to C I/O commands like printf
• Computer on null modem cable is a PIC terminal
• Uses only two pins, C6 and C7
• Called the Master Synchronous Serial Port (MSSP)
• Coupled with memory-mapped UART
• Three timers
• 14 interrupts
• Two capture/compare/PWM (CCP) modules

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How Its Done

• The hardest part is the PWM setup
• The PWM uses a counter, Timer 2, to set a PWM
  period
• Timer 2 counts the processor clock pulses
• The output pulse is ON for a specified number of
  clock pulses
• The duty cycle is the ratio of the number of ON
  pulses to the total period set by Timer 2
• The rest is easy
• ADC is 10 bit, from any of 5 pins of bus A
• Pushbuttons are read from three bits of bus C
• H-bridge word is made up from
• PWM outputs
• Bits that tell us forward-backward, up-down,
  toggled by pushbuttons
• Magnet drive is logic output to bit of bus C,
  toggled on-off by pushbutton

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Your Resources

• The CCS PIC C Compiler MCU
• Only for mid-range PIC microcontrollers
• Other compilers for high-end PIC microcontrollers
• The file 16F876A.h in your compiler
• The PIC Project Board Programmer-Debugger
• Available in room 237
• Power supplies and lab equipment help you
  integrate your project
• Books
• The PIC MCU C Compiler Reference Manual, comes
  with the compiler
• The C Programming Language, 2nd Edition Kernighan
  Ritchie, Prentice Hall (1988), ISBN-10
  0131103628, ISBN-13 978-0131103627, about 40
  from Amazon
• PIC micro MCU C, Nigel Gardner, about 15 from
  Microchip, Inc.

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The PIC PWM

• Based on Timer 2
• Three timer stages
• First stage divides main clock by 1, 4, or 16
• Second stage divides by user-specified number
• Third stage divides by 1-16 and resets timer
• Total PWM period (1/frequency) is total count
• PWM operates by turning on an output pin for a
  user-specified number of main clock ticks

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Timer 2 Setup

• User call in CCS C setup_timer_2(T2_DIV_BY_nn,
  period, postscale)
• The nn may be 1, 4, or 16
• The period is 0 to 255
• The postscale is 1 to 16

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How the PWM Controls Power

• The PWM has a cycle of T2_Ticks clocks
• Use a C call set_pwmn_duty(d_clocks)
• The number n may be 1 or 2
• Each cycle is ON for n clocks
• The PIC 16F876A ADC is 10 bits
• Scale ADC output so that full scale is T2_Ticks
• Thus T2_Ticks must be 210 1024 or greater

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One Way for Timer 2 Usage
Processor Clock of 4 MHz, period is 255,
postscale is 1
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The H-Bridge Driver

• PWM output is read back into the processor
• PWM output is the drive signals for two of the
  four H-bridge MOSFETs
• The other two signals are zero
• Which are used depends on direction of motor
• PWM bits are put into proper position in control
  byte with shifts
• Combine to form 8-bit H-bridge drive word
• Output on pins B0 through B7

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The Scratchy Button Contact

• A pushbutton can give a scratchy waveform
• One solution
• Keep track of what the last pushbutton signal was
• Log a button push only when you see it change
  from unpushed to pushed on a pass through the
  program
• Hardware H-bridge allows a loop delay
• Build the H-bridge drive word with external
  hardware to decouple the processing loop speed
  from the PWM waveform
• Add a delay of a few milliseconds at the end of
  the loop
• Will see only one button push as the button makes
  contact
• Use a Schmidt trigger on each pushbutton
• Keep a longer track record for pushbutton bounce
  logic

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Structure of the Program

• Context
• include lt16F876A.hgt
• define, device and use statements
• Setup
• Calls to PIC-specific functions in CCS PIC C
• Set up ADC,s PWM, I/O ports
• Loop
• Read the thumbwheels
• Set the PWM duty cycles
• Read the pushbuttons and toggle forward-back,
  up-down, magnet
• Make the H-bridge word and write it
• Pause?

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Pin-out of PIC 16F876A
Pins Used in Project Shown in Red
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Things to Do to Make It Work

• Implement hardware H-bridge drive, OR
• Work out the pin-outs to pushbuttons, H-bridge as
  connected
• Make the initial toggles what you expect
• Forward-backward
• Up-down
• Magnet on-off
• Change the program, not the wires
• Software H-bridge drive may be used for a
  prototype may be smooth enough to use

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The Initial State

• What is the crane doing when the power is
  applied?
• Forward-back is forward
• Up-down is down
• Magnet is off
• What about the thumbwheels?
• If they are turned up, the crane and lift will
  move
• You can add logic to keep things off until both
  thumbwheels are zero.

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What About Pushbutton Bounce?

• Some elementary logic is already there
• Toggles forward-back, etc. only when pushbutton
  transitions from un-pushed to pushed between
  loops
• If the loop is fast and the button is slow, this
  can happen more than once
• One solution Add a delay in the loop
• The controls only need to be read 10 to100 times
  a second
• Experimentation may give you a good delay number
  that provides robust key bounce performance with
  the pushbuttons
• Another solution
• Keep a history of several pushbutton outputs
• Average them or perform logic to provide robust
  determination of when to toggle the bits

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The C Program Code

• Environment
• Include the processor definitions for the 16F876A
• Define the constants
• Specific compiler directives
• Initialization
• Declare all the variables and initialize them
• Set up ADCs, PWM, and I/O
• Processing loop
• Read the thumbwheels and pushbuttons
• Formulate the H-bridge driver outputs
• Output the H-bridge and magnet drive outputs
• Repeat

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Environment
include lt16F876A.hgt fuses HS,NOWDT,NOPROTECT,NOL
VP device ADC10 //10 bits right justified in a
16 bit word use delay(clock10000000) //Put your
clock rate here 4 MHz 4000000 //use
rs232(baud9600, xmitPIN_C6, rcvPIN_C7) //RS-232
not used define scale_shift 4 //log2(T2_Ticks/2
(ADC bits)) see below define speedpos
0 //Propulsion is bottom 4 bits define liftpos
4 //Lift is top 4 bits
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Initialization Variable Declarations
void main() long int speed, lift, adc_out
//long int is 16 bits in CCS PIC C MCU compiler
int speed_Hbridge, lift_Hbridge, Hbridge,
lsb //8 bits short forward, lift_up,
magnet_on //One bit short pbp, pbp_state,
pbl, pbl_state, pbm, pbm_state //Anti-bounce
logic
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Initialization Set Up I/O, ADCs
//SETUP SET_TRIS_B(0b00000000) //B pins are
H-bridge driver -- all outputs
SET_TRIS_C(0b11100000) //Buttons input on C7,
C6, C5, magnet drive on C4 setup_adc_ports(
ALL_ANALOG ) //Inputs are A0 A1 A2 A3 A5, ref is
5 V setup_adc( ADC_CLOCK_INTERNAL
) //Internal clock, or box crystal if there
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Initialization PWM Setup
//Set up PWM clock, which is always timer
2 //PWM frequency determination //We will try
244 Hz. If it is too low and motor buzzes, try
977 Hz setup_timer_2(T2_DIV_BY_16, 255,
1) //The ADC output must be scaled so that
2101024 is scaled to the PWM period //See
"define scale_shift 4" above
setup_ccp1(CCP_PWM) //Configure CCP1 as a PWM
setup_ccp2(CCP_PWM) //Configure CCP2 as a PWM
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Initialization Variable Initialization
//VARIABLE INITIALIZATIONS pbp1 //Initialize
all pushbuttons as off, with pullups pbl1
pbm1 pbp_state1 pbl_state1
pbm_state1 forward1 //Initially, move
forward lift_up0 //Initially, hook moves
down magnet_on0 //Initially, magnet is off
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Processing Loop Read Thumbwheels
do //Read speed thumbwheel
set_adc_channel(0) //Propulsion thumbwheel is
port A0 delay_us(10) //A small delyay is
required before a read adc_outRead_ADC()
lsbbit_test(adc_out,0)//Extend LSB on
scaling shift speed ((adc_outlsb)ltltscale_
shift)-lsb set_pwm1_duty(speed) //Output
thumbwheel data to PWM for propulsion
speed_Hbridgeinput_state(PIN_C2) //Set drive
for first MOSFET speed_Hbridge(speed_Hbrid
ge)ltlt1 //Second MOSFET
Propulsion thumbwheel shown lift thumbwheel
similar
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Processing Loop Read Pushbuttons
//Check push button for propulsion direction
reversal pbpinput_state(PIN_C7) //Read
propulsion pushbutton (0 pushed)
forward(!pbp pbp_state) //Toggle if
transition from 1 to 0 pbp_statepbp
output_bit(PIN_C3,forward) //Put propulsion
toggle on pin C3 //Check push button for crane
lift direction reversal pblinput_state(PIN_
C6) lift_up(!pbl pbl_state)
pbl_statepbl output_bit(PIN_C0,lift_up)
//Put lift toggle on pin C0 //Check magnet
push-on, push-off button pbminput_state(PIN
_C5) magnet_on(!pbm pbm_state)
pbm_statepbm output_bit(PIN_C4,magnet_on)
//Put magnet toggle on pin C4
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Processing Loop Build H-Bridge Control, Output
Drive Signals
//Build and output H-bridge output word //This is
probably too slow for smooth motor speed
control //because the waveform update granularity
is the loop time, //not the Timer 2 comparator,
so a hardware solution is best.
speed_Hbridge (!forward) ? speed_Hbridge
speed_Hbridgeltlt2 lift_Hbridge (!lift_up)
? lift_Hbridge lift_Hbridgeltlt2 Hbridge
(speed_Hbridge ltlt speedpos) (lift_Hbridge ltlt
liftpos) output_B(Hbridge) //Output the
H-bridge bits to bus B while (TRUE)
Far too slow for 10 bit accuracy
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Issues

• Use of processing loop to update PWM waveform
• Slower than Timer 2 granularity
• 10 bits accuracy will not be achieved
• Accuracy of 5 bits is about 3 and may be enough
  if that is achieved
• Data is available to provide a hardware solution
• Key bounce logic is just a start
• Will probably need more
• Hardware solution for H-bridge drive will allow
  adding a delay at the end of the loop
• Software solution will require keeping track of
  the last several pushbutton inputs and taking an
  average, or similar logic

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Other Things You Can Do

• Add a pushbutton that stops everything
• Add logic to program
• Reset the PIC microcontroller
• Make the forward-back and up-down pushbuttons
  up-stop-down instead
• Add a button that is pushed when the crane gets
  to the end of the rails that stops the
  propulsion, or reverses it
• Make the relationship between the thumbwheel and
  the motors something other than linear to improve
  control and feel
• Whatever you can think of

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Summary

• You have your project ready to go
• Hand unit
• Crane motors
• PIC and H-bridge boards
• Electromagnet
• Put your PIC board on a programmer-debugger in
  room 237
• Run the program and wring out the glitches
• Use your own ideas to improve the program and
  your project